(1) Field of the Invention
This invention relates to methods of forming colored patterns on the surfaces of aluminum or its alloys without using any dyes or pigments and to an aluminum article having a colored pattern of wood grain.
(2) Description of the Prior Art
Hitherto, there have been known many methods of forming colored patterns on the surfaces of aluminum or its alloys. In prior methods, as disclosed in Japanese Patent Publication No. 3895/77 and 4616/75 and Japanese Patent Application laid open to public inspection No. 41735/75, a resist film is applied to or printed on an aluminum work piece to form protected areas in conformity with a desired pattern, the work piece is then subjected to an anodic oxidation to form a barrier-type oxide film or to a chemical conversion to form a chemically oxidized film and, after removal of the resist film, to the second anodic oxidation or chemical conversion to form a pattern of a colored film, namely, the prior methods include the steps of: resist pattern printing, primary anodic oxidation, (stopping-up of pores), removal of resist film, secondary anodic oxidation (electrolytic coloring); or, resist pattern printing, chemical oxide film formation, removal of resist film, chemical conversion (chemical formation of colored oxide film). Another method which also includes a printing process for patterning is disclosed in Japanese Patent Publication No. 21022/76 which comprises applying a TFS coating to the surface of an aluminum work piece, applying a pattern coating thereon by means of screen or off-set printing and drying and baking together the TFS coating and the patterning ink. These methods which include a printing process for patterning, however, have a shortcoming that the printing process is expensive and takes much time and, consequently, results in decrease in the mass productivity or productivity of these methods and, in addition, the printing process makes it difficult to produce a variety of patterns each at relatively small produce.
As a method which includes no printing process, Japanese Patent Application laid open to public inspection No. 60244/77 discloses a method of electrolytically coloring aluminum to form a pattern of wood grain which comprises subjecting an aluminum work piece to electrolysis in an alkaline electrolytic bath added with a barrier-type oxide film forming electrolyte by means of an alternating current or a current exhibiting the same effect with an alternating current. Japanese Patent Application laid open to public inspection Nos. 3535/77, 61139/77 and 70951/77 disclose methods of forming patterns on aluminum surfaces by electrolytic coloring through controll of electrolytic formation of a barrier layer after anodic oxidation. However, these methods in which colored patterns are formed by electrolytic coloring after modification of the thickness of the barrier layer are unsuitable for work piece having complicated shapes and poor in productivity because of difficulty in modification of the thickness of the barrier layer. On the other hand, the method disclosed in Japanese Patent Application laid open to public inspection No. 60244/77 is applicable to work pieces having complicated shapes, though the patterns formed in this method are lengthwise extending short etching figures which are somewhat similar to but far apart from the straight grain of natural wood and it is impossible to form patterns imitating the cross or flat grain of wood.